In recent years the method or mode of operating fuel burners has been altered because of the escalating cost of fuel. Until recent years, fuel burners, particularly oil burners, were operated with an intermittent source of ignition, a source of combustion air, and the continuous monitoring of a flame by a sensor. The sensor normally was a cadmium sulfide type of cell. The control devices themselves generally were large and utilized electro-mechanical components.
In the older types of fuel burners or oil burners, the operation of the ignition source occurred whenever fuel was introduced into the combustion chamber. This type of operation was generally considered as quite safe as there was little chance of the flame going out and there being no source of ignition to reignite the fuel. Also, there was little or no problem with the photocell or sensor being fooled by a hot refractory wall of the oil burner. The overall monitoring of the operation of the system relied both on the operation of the photocell and on a safety switch which ultimately would remove the power to the source of oil and ignition in the event of the loss of a flame. The loss of a flame was normally sensed by the photocell and even though the photocell had a relatively slow response time, the source of ignition was still "on" to prevent any build up of oil.
In order to accomodate for the higher operating costs, fuel burners of the oil burner type are now more commonly operated with an interrupted source of ignition. The quality of the fuel being used now varies considerably, as opposed to a more uniform quality of fuel that was available a number of years ago. This variation in fuel quality and the interrupted operation of an ignition source provides a potential for the loss of flame which is less stable under present operating conditions than under the older operating conditions. In the event of the loss of a flame when the ignition source has been turned "off", the photocell requires a short period of response time. This response time can be extended or exaggerated by a hot refractory wall of the burner. During such a loss of flame when no ignition source is present and with a hot refractory wall present, the oil burner might introduce oil that was not properly ignited and create an unsafe condition before a safety switch caused the shut down of the burner. In order to overcome this unsafe operating mode for an oil burner, it has become necessary to improve the response time to the photocell that is used to sense the existance of flame.